docs: histogram.txt: convert it to ReST file format

Despite being mentioned at Documentation/trace/ftrace.rst as a
rst file, this file was still a text one, with several issues.

Convert it to ReST and add it to the trace index:
- Mark the document title as such;
- Identify and indent the literal blocks;
- Use the proper markups for table.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

kernel/trace/Kconfig

@@ -605,7 +605,7 @@ config HIST_TRIGGERS
 	  Inter-event tracing of quantities such as latencies is also
 	  supported using hist triggers under this option.
 
-	  See Documentation/trace/histogram.txt.
+	  See Documentation/trace/histogram.rst.
 	  If in doubt, say N.
 
 config MMIOTRACE_TEST